A&A 611, A34 (2018) https://doi.org/10.1051/0004-6361/201630134 Astronomy & © ESO 2018 Astrophysics
A survey for low-mass stellar and substellar members of the Hyades open cluster Stanislav Melnikov1,2, and Jochen Eislöffel1
1 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany e-mail: [email protected] 2 Ulugh Beg Astronomical Institute, Astronomical str. 33, 700052 Tashkent, Uzbekistan
Received 25 November 2016 / Accepted 11 November 2017
ABSTRACT
Context. Unlike young open clusters (with ages < 250 Myr), the Hyades cluster (age ∼ 600 Myr) has a clear deficit of very low-mass stars (VLM) and brown dwarfs (BD). Since this open cluster has a low stellar density and covers several tens of square degrees on the sky, extended surveys are required to improve the statistics of the VLM/BD objects in the cluster. Aims. We search for new VLM stars and BD candidates in the Hyades cluster to improve the present-day cluster mass function down to substellar masses. Methods. An imaging survey of the Hyades with a completeness limit of 21m. 5 in the R band and 20m. 5 in the I band was carried out with the 2k × 2k CCD Schmidt camera at the 2 m Alfred Jensch Telescope in Tautenburg. We performed a photometric selection of the cluster member candidates by combining results of our survey with 2MASS JHKs photometry. Results. We present a photometric and proper motion survey covering 23.4 deg2 in the Hyades cluster core region. Using optical/IR colour-magnitude diagrams, we identify 66 photometric cluster member candidates in the magnitude range 14m. 7 < I < 20m. 5. The proper motion measurements are based on several all-sky surveys with an epoch difference of 60–70 yr for the bright objects. The proper motions allowed us to discriminate the cluster members from field objects and resulted in 14 proper motion members of the Hyades. We rediscover Hy 6 as a proper motion member and classify it as a substellar object candidate (BD) based on the comparison of the observed colour-magnitude diagram with theoretical model isochrones. Conclusions. With our results, the mass function of the Hyades continues to be shallow below ∼0.15 M indicating that the Hyades have probably lost their lowest mass members by means of dynamical evolution. We conclude that the Hyades core represents the “VLM/BD desert” and that most of the substeller objects may have already left the volume of the cluster.
Key words. stars: low-mass – open clusters and associations: individual: Hyades – brown dwarfs – stars: luminosity function, mass function
1. Introduction evolution the lowest mass members may escape from a cluster core (“evaporate” from the cluster) due to dynamical encoun- The accurate initial mass function (MF) of Galactic open clusters ters and mix-up with the field objects (Terlevich 1987; Kroupa allows us to build up a picture of the initial conditions of cluster 1995; de La Fuente Marcos & de La Fuente Marcos 2000). formation and to investigate their further evolution. The bright Therefore, detection of these objects will be more complicated. end of the mass function has been analysed in many detailed De La Fuente Marcos & de La Fuente Marcos(2000) suggested studies of bright clusters. In the last decades, on the other hand, that this effect can already be noticeable in the clusters with ages deep photometric surveys of open clusters were focused on the of ≥200 Myr. The first surveys of the intermediate-age clusters, faint MF end reaching out to the lowest mass members. The which covered only a small percent of the cluster core areas, nearby open clusters are very convenient targets for this goal. did not find any significant population of low-mass members, In the solar neighbourhood there are a number of open clus- similar to what was found for the Pleiades. The first studies of ters including the Pleiades, the Hyades, Praesepe (M44), and the the Coma open cluster (age ∼ 500 Myr) showed that this clus- Coma Berenices open cluster (Melotte 111). Extensive deep sur- ter has a deficit of low-mass stars in comparison to the younger veys of the Pleiades (age ∼ 120 Myr) have led to the discovery of clusters (Deluca & Weis 1981; Odenkirchen et al. 1998). The a large population of very low-mass stars (VLM) and substellar recent and deeper surveys of this cluster covered a larger field members known as brown dwarfs (BDs) (Stauffer et al. 2007; and were reaching into the substellar domain. They confirmed Lodieu et al. 2012a; Zapatero Osorio et al. 2014; Bouy et al. that this deficit seems to be intrinsic and this finding (Casewell 2015). A considerable low-mass population has also been discov- et al. 2006; Melnikov & Eislöffel 2012) supports the idea that ered in some other younger stellar clusters (α Per: Lodieu et al. the depletion is caused by dynamical evolution. Recent studies 2012b; σ Ori: Peña Ramírez et al. 2012). These surveys showed of Praesepe (∼600 Myr), another open cluster of similar age, that the initial conditions of star formation in stellar clusters were based on the analysis of all-sky surveys UKIDSS (Boudreault effective in creating low-mass members. et al. 2012) and 2MASS, PPMXL, Pan-STARRS (Wang et al. However, the proximity of a cluster also has the disad- 2014) found conflicting results. Boudreault et al.(2012) found vantage of a large extension on the sky, which renders sur- that the Praesepe MF is consistent with that of the Galactic veys for cluster membership difficult. Moreover, with cluster disk population down to 0.07 M , whereas Wang et al.(2014)
Article published by EDP Sciences A34, page 1 of 14 A&A 611, A34 (2018) concluded that the cluster MF shows a deficit of members below 0.3 M . Therefore, deep wide-field surveys of intermediate- aged open clusters (with ages of 450–600 Myr) are required to improve the comparison of their mass functions with those of the younger clusters and construct a more reliable scenario of how open clusters evolve with age. The Hyades open cluster (d = 46 pc) with age ∼ 600 Myr (age = 625 Myr, Perryman et al. 1998), which has a Pleiades- like stellar population, is one of the most studied open clusters located in the solar neighbourhood. The earliest wide-field search for Hyades members covering ∼110 deg2 discovered a deficit of M-type dwarfs in this cluster (Reid 1992, 1993) with respect to the solar neighbourhood mix. From these observations Reid(1992) derived a gravitational binding radius of ∼10.5 pc and a total cluster mass of 410–480 M . Deeper imaging sur- veys, which covered only small areas to reach fainter members did not lead to a discovery of any significant population of such objects (Reid & Hawley 1999; Reid & Mahoney 2000; Gizis et al. 1999). A survey covering 10.5 deg2 by Dobbie et al.(2002) also failed to find any new low-mass members and recovered only the known stellar member RHy 29 (Reid 1993). A wide-field study of the Hyades based on the recent proper motion cata- logue PPMXL, 2MASS, and Carlsberg Meridian Catalogue 14 (CMC14)1 photometry has enabled a full census of the kine- matic cluster members down to masses of ∼0.2 M in a region up to 30 pc from the cluster centre (Röser et al. 2011). Com- bining these three surveys, Röser et al.(2011) carried out a three-dimensional analysis of the cluster population and found Fig. 1. Area of the Hyades cluster mapped by our TLS imaging sur- that the Hyades have a tidal radius of ∼9 pc with clear out- vey. The total size of the surveyed area is ∼23.4 deg2. Star symbols are ward mass segregation. Using the PPMXL and Pan-STARRS1 photometrically selected Hyades member candidates, which are listed sky surveys Goldman et al.(2013) analysed the same area of the in Table A.1. Hyades and pushed the census of its members down to 0.09 M . Based on the deepest survey over 16 deg2 of the Hyades core, Bouvier et al.(2008) reported the discovery of the first two 2. TLS photometric survey and basic CCD BDs and confirmed membership of 19 low-mass stellar mem- reduction bers. Analysing the statistically significant number of Hyades We have carried out a new wide and deep imaging survey of members found in the previous studies, Bouvier et al.(2008) the Hyades open cluster (RA = 4h26m Dec = +15◦ 00) in the RI concluded that the present-day mass function of the Hyades is bands using the 2k × 2k CCD camera in the Schmidt focus of the clearly deficient in the VLM/BD domain compared to the initial 2 m Alfred Jensch Telescope in Tautenburg. A short description MF of the Pleiades, which have a similar population structure, of the RI photometric system of the CCD camera can be found but are much younger than the Hyades. Another study based on in Melnikov & Eislöffel(2012). The Hyades are a very close the all-sky surveys UKIDSS and 2MASS (Hogan et al. 2008) and open cluster and its core area (r ∼ 2.8 pc; Perryman et al. 1998) following spectroscopic analysis (Casewell et al. 2014; Lodieu extends over a very large sky area of ∼50 deg2. Our photometric et al. 2014) added several BDs to the substellar domain of the survey obtained in October–November 2006 covers 23.4 deg2 in cluster. Nevertheless, the updated MF still shows the apparent its central area, i.e. about 47% of the cluster core (see Fig.1). deficit of the lowest mass members (Lodieu et al. 2014). The total exposure time per filter (one frame) was 600 s, and In this paper, we present the results of a new deep imaging for this exposure time the limiting magnitude of the frames was survey of the Hyades open cluster obtained with the wide- estimated to be 22.5 in the I band. To avoid saturated stellar field Schmidt camera at the 2 m Alfred Jensch Telescope of profiles, we ensured that I-magnitudes were in general >14.5. the Thüringer Landessternwarte in Tautenburg, Germany (TLS). In total, 65 fields of good quality were obtained in the course Details of the photometric observations and data reduction are of this survey. The positions of the field centres were chosen described in Sect.2. In Sect.3 we introduce the photometric so that each field overlaps with its neighbours by ∼12s (∼30) in selection procedure of VLM objects and BD candidates; we then right ascension and ∼3.05 in declination. The overlapping regions compute proper motions of our optically selected candidates by allowed us to check the quality of our photometric calibration combining the TLS astrometric calibration with earlier epoch by comparing the magnitudes of stars located in the overlapping all-sky surveys, and describe the results. In Sect.4 we report areas of the adjacent frames. on the comparison of our photometric selection with the results The raw images were reduced following standard recipes of previous surveys of the Hyades; we also discuss the updated in IRAF2; this procedure included overscan correction, bias mass function of the cluster combining our new cluster member subtraction, and dome flat-fielding. The I-band images contained candidates with those from the previous studies. a prominent interference fringe pattern caused by night sky
1 Copenhagen University Observatory, Institute of Astronomy, 2 IRAF is distributed by National Optical Astronomy Observatories, Cambridge, UK, & Real Instituto Y Observatorio de La Armada, which is operated by the Association of Universities for Research in F. E. S., 2006, VizieR Online Data Catalog: I/304. Astronomy, Inc., under contract with the National Science Foundation.
A34, page 2 of 14 S. Melnikov and J. Eislöffel: The VLM/BDs in the Hyades cluster emission. These fringe strips were removed with a fringe mask cool BD atmospheres. The BT-Settl models, based on a solar constructed from the whole set of the I-band images. The R- abundance from Caffau et al.(2011, CIFIST2011) were already and I-band images were then aligned where necessary and all employed for the Hyades member selection in Goldman et al. were astrometrically calibrated using the Graphical Astronomy (2013), who used the wide global surveys such as Pan-STARRS1 and Image Analysis Tool software (GAIA) and the Hubble Guide and SDSS combined with 2MASS and WISE infrared survey. Star Catalog (GSC v.1.2) as reference. The GSC contains posi- The BT-Settl model grid allows a good reproduction of near- tions for most of the field stars down to magnitude V = 16. Each infrared (NIR) spectral energy distribution of cool VLMs and of our Hyades CCD frames contains more than 30 reference stars BDs (Allard 2014). The current BT-Settl model grid (Baraffe evenly distributed across the field, and the accuracy of the astro- et al. 2015; Allard 2016) covers the stellar parameter range for 00 metric solution for individual images is better than 0. 4 in both the low-mass objects with Teff = 1200–7000 K, and therefore filters for both equatorial coordinates. the models are valid for both the lowest mass stars and BDs. Cosmic ray hits (cosmics) were removed by combining each We decided to utilise the latest BT-Settl model grid for our pair of RI frames of the same sky field into one image and membership analysis. rejecting cosmics. The images also had several different kinds To distinguish the Hyades member candidates from fore- of artefacts and extended objects which had to be discriminated ground dwarfs, we used the BT-Settl isochrones (solid curves in from star-like objects. A description of these various artefacts Fig.2) calculated for the Hyades age (625 Myr). For the photo- and the method allowing us to clean the images is described in metric selection of the candidates, we first took into account the Melnikov & Eislöffel(2012). cluster depth; the cluster core has a radius of 2.7 parsec, which Instrumental magnitudes of all extracted sources were then means the objects can be 0m. 12 fainter or brighter than the central extracted based on the measurement of the point spread func- cluster isochrone. Moreover, the main sequence of the Hyades in tion (PSF) using the daophot package of IRAF. Finally, we the R − (R − I) CMD constructed from the previously known converted the instrumental magnitudes into RI-band magnitudes members from Reid(1993) shows that it is not just a thin line, using photometric standards observed in Landolt Selected Areas but has a width of ∼0.2–0.3 mag, which cannot be explained (Landolt 1992). Photometric errors for the RI bands depend on as resulting from the photometric errors or the cluster depth. magnitude: in the range of 14–18 the errors gradually increase Reid(1993) analysed this effect on MV − (V − I), but it is also m m from 0. 01 to 0. 04, but for objects with R > 18 and I > 18 observable in MI − (I − K). Reid(1993) suggested that this effect the errors grow faster and reach about 0m. 3 for R ≈ 20. For the can be a sequence of natural dispersion of stellar parameters, I − (I − J) and I − (I − K) CMDs, the I-errors are combined where the high rate of unresolved binaries amongst the cluster with the infrared photometry errors provided in the 2MASS members (Griffin et al. 1988; Reid 1993) can be partly responsi- catalogue. ble for this scatter. Thus, we used the additional colour strip of 0.15 mag width in order to take into consideration this disper- sion and increase the detection probability of real members. As 3. Selection of very low-mass stellar and brown a result, we started with all objects within a strip that takes into dwarf candidates account the cluster depth, the dispersion of colour indices, plus a 1.5σ wide strip due to photometric uncertainty (PSF photomet- All 65 CCD cluster fields together contain about 290 000 objects ric errors only). The strips are shown in the I − (R − I) CMD that were detected by SExtractor (Bertin & Arnouts 1996) in in Fig.2 as the two dashed curves on both sides of the BT-Settl the R, I bands. We plot the I − (R − I) colour-magnitude dia- isochrone which are getting wider due to increasing photometric gram (CMD) for the extracted sources (Fig.2) and compare errors with growing magnitude. their diagram position with the model isochrones for low-mass When we use the BT-Settl model for analysis of the I −(R− I) objects, shifted to the distance of the Hyades (m − M = 3.33, CMD, we can roughly split the procedure into two parts. For the Perryman et al. 1998). Interstellar reddening towards the Hyades bright objects with I < 18, the BT-Settl isochrone is separated is very low (Taylor & Joner 2002) and can be neglected. Pre- from the bulk of the field dwarfs quite well and we have found vious studies of the Hyades members exploited mostly the only several tens of photometric candidates within the photomet- NextGen (Baraffe et al. 1998), DUSTY (Chabrier et al. 2000), ric errors. There are also a number of objects located redward of and COND (Baraffe et al. 2003) isochrones (e.g. Dobbie et al. our red uncertainty boundary which we included in our initial 2002; Bouvier et al. 2008). NextGen evolutionary models for sample, especially in the upper part between I = 14–16.5. Some solar metallicity based on non-grey dust-free atmosphere mod- of these reddish objects at the CMD top could be background els described various observed properties of M dwarfs down to dwarfs, distant galaxies, or red giants. The observed displace- the bottom of the main sequence (CMDs, spectral types, etc.), ment towards the red could be partly caused by the depth of the whereas the DUSTY and COND models try to reproduce the cluster and by the binarity of the objects. In the most extreme same properties for BDs, taking into account the possible for- case, a binary consisting of equal mass stars may be lying up mation and opacity of dust grains in the atmosphere of objects to 0.75 mag above the single star sequence. Since we cannot with Teff . 2800 K. The DUSTY and COND models are differ- identify the origin of this reddening using only the I − (R − I) ent in that the latter include effects of rapid gravitational settling CMD, all these reddish objects were initially included in our list of the grains in the lower atmospheric layers below the photo- as potential cluster members. As a result, we identified several sphere. Chabrier et al.(2000) predict that this process will occur tens of low-mass cluster member candidates in the magnitude at a temperature of Teff . 1300 K, which corresponds to a mass range 14 < I < 18, covering from 0.15 M to about 0.075 M . of M ≈ 0.04 M . The BT-Settl models (Allard et al. 2012) are These sources were then cross-identified with JHK 2MASS- a further development of evolutionary models which account photometry and were subjected to further checks, which are for the formation of dusty clouds via a parameter-free cloud detailed in the following. model (based on the cloud microphysics from Rossow 1978). For objects with I > 17.5, the BT-Settl model predicts that Compared to DUSTY, the BT-Settl models include, among other the R − I isochrone has a turnover and fainter objects will have microphysical processes, gravitational settling of the dust in the bluer colour indices. Therefore, for the candidates with masses
A34, page 3 of 14 A&A 611, A34 (2018)
VLM/BD candidates 14 PM candidates BT-Settl 2015 BT-Settl 625 Myrs photom. uncertainty 0.15 15 BDs boundary Lithium limit 15 0.13 0.11 0.10 0.11 16 0.10 0.09 16 0.09 17 0.08 I I 17 0.08 18
0.07 18
19 0.07
19 0.06 20
20 0.5 1 1.5 2 2.5 3 3.5 1.5 2 2.5 3 3.5 4 R-I I-J
14 BT-Settl 2015 BT-Settl 2015 0.15 13 0.13 Fig. 2. Top left: I − (R − I); top right: I − (I − J); 15 0.13 0.11 bottom left: I − (I − K); and bottom right: J − 0.11 0.10 − 0.10 (J K) colour-magnitude diagrams of optically 14 0.09 16 selected candidates (points) from the TLS and 0.09 0.08 2MASS/UKIDSS surveys; the proper motion can- didates are indicated by filled circles. In all CMDs,
I 15 17 J we show 625 Myr BT-Settl isochrones shifted to 0.08 0.07 the distance of the Hyades cluster (m − M = 3.33) as solid lines. The vertical line labelled with stel- 18 16 0.06 lar mass (M ) is the mass scale according to this
0.07 model. The two dashed lines outline the selec- tion area (see text). The upper horizontal line at 19 17 I = 18.1 shows the stellar/substellar boundary and the lower line indicates the lithium burning limit.
20 The BT-Settl isochrone in the J − (J − K) CMD
2.5 3 3.5 4 4.5 5 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 agrees well with the colour indices of the faintest I-K J-K member candidates. below 0.075 M , one can see that the model coverage signif- J − (J − K) isochrone is almost vertical for J < 14.5, but for icantly overlaps with the field dwarfs at the I − (R − I) CMD fainter J the colour index becomes considerably redder. This (Fig.2). Using the BT-Settl isochrone has led to a selection behaviour agrees well with the colour indices of the faintest of several thousand objects. These sources were then cross- member candidates, which show very red colour indices on this identified against sources in 2MASS and the UKIRT InfraRed CMD (Fig.2). Deep Sky Survey (Galactic Clusters Survey, UKIDSS). The Goldman et al.(2013) note that the discrimination between UKIDSS survey has a deeper detection limit, but its photometric the Hyades cluster sequence and the bulk of the background selection contained only the K band for the Hyades. We discuss stars is better when the wavelength difference between the bands this photometric selection in the following section. is greater. Thus, the I − (I − K) CMD is a good one for this preliminary discrimination. First, we cross-identified all our ini- tial candidates selected from the I − (R − I) CMD in the Two 3.1. Two Micron All-Sky Survey selection Micron All-Sky Survey (2MASS; Skrutskie et al. 2006). Using a 00 As the next step of the photometric selection, we used three matching radius of 2. 5, we derived JHKs photometry for all our NIR CMDs of I − (I − J), I − (I − K), and J − (J − K) (see candidates up to I ≈ 19.3. We combined the derived NIR pho- Fig.2). As for the I − (R − I) diagram, the 625 Myr BT-Settl tometry with our I-magnitudes into the three additional CMDs isochrones from Allard(2016) are shown on these IR CMDs (see Fig.2). For our targets with I & 19.3, the 2MASS survey in Fig.2 as solid lines. The two dashed lines at both sides did not contain IR photometry and thus we cross-identified these of the isochrones indicate the selection area defined by param- against sources in the UKIDSS survey. Although the Hyades eters adopted for I − (R − I) CMD such as the photometric are covered by this survey, it provides only K-magnitudes (K1) uncertainty, the cluster depth, and the natural photometric dis- for this cluster. Finally, we used the transformation equation persion. All CMDs in Fig.2 also represent a scale of stellar from Hewett et al.(2006) to convert UKIDSS K1 magnitudes mass according to the model. Using the CMDs, we then selected to 2MASS Ks. all the photometric candidates which agree with the theoreti- The analysis of the large candidate set on the I − (I − K) cal NIR isochrones within defined selection areas. Contrary to CMD showed that almost all UKIDSS and 2MASS sources with I −(R− I), the BT-Settl isochrones in the I −(I − J) and I −(I − K) I > 17.5 have I − K bluer than is predicted by the BT-Settl CMDs predict a gradual increase in the colour indices with I − (I − K) isochrone, and thus they are probably field dwarfs. decreasing stellar mass at least down to 0.05 M . The BT-Settl Therefore, we excluded all the objects from the further analysis.
A34, page 4 of 14 S. Melnikov and J. Eislöffel: The VLM/BDs in the Hyades cluster
In our results, we identified only seven photometric candidates with I > 17.5. For I < 17.5 we selected several tens of candidates and rejected those candidates which fail our criterion in any of the other NIR CMDs. As a result, all candidates which have only UKIDSS photometry are were rejected.
3.2. Field object contamination and background giant stars An estimate of the number of contaminating field stars can be obtained from the Besançon Galaxy model (Robin et al. 2003), which gives star counts depending on their brightness, colour, and Galactic coordinates. Using this model, Goldman et al. (2013) have found that the contamination by field stars is neg- ligible (<10%) up to 18 pc of the cluster centre. Since our survey lies within this radius, we will not consider contamination as essential for our conclusions. Finally, we used the (J − H) − (H − K) diagram to weed out possible background giants from our sample. This type of con- taminating objects probably experiences interstellar extinction and tends to populate the top left side of the diagram above the sequence of the dwarf stars (Goldman et al. 2013). In Melnikov & Eislöffel(2012) we tried to find giants as background con- taminants projected onto the Coma open cluster with the help of the analysis of narrowband spectral indices (Jones 1973), which Fig. 3. PM diagram of photometrically selected objects in the Hyades allowed us to distinguish genuine low-mass members from the region as a vector diagram of µα cos δ and µδ. The grey box plotted fol- far red giants with similar colour indices. However, no giants lowing Bouvier et al.(2008) and Bryja et al.(1994) defines the region of were found in the background of the Coma cluster. The Hyades PMs expected for the cluster. Taking into account rms errors, 14 objects (at l = 180, b = −22.3) are located in the opposite direction of 66 optically selected candidates (empty circles) were classified as PM to the Galactic centre and quite high above the Galactic plane. members (filled circles). Therefore, we do not expect to find a large amount of distant whereas for 8 objects, SpT (2MASS) values are later by two sub- background M-type giants in the direction of the Hyades either, classes and larger than those derived from the TLS photometry. given that this cluster is located at high Galactic latitude as well. We should note that this luminosity–spectral calibration As a result of this NIR two-colour analysis, we found nine sequence is based on the stellar SEDs of VLMs only and is valid objects that are located in the CMD region with high extinction. for stellar objects with K < 13.8 (masses 0.08 M ). Therefore, To double check, we calculated proper motions of these targets & this calibration method may not be reliable for BD candidates (see Sect. 3.4). Most of these objects have proper motions around and it may not provide the proper spectral classification for zero which seems to favour the idea that they might be back- objects with SpT about and later than L0. ground objects. Finally, we excluded these objects from our list of photometric VLM candidates. 3.4. Proper motion as a selection criterion
3.3. Spectro-photometric classification The mean proper motion (PM) of the Hyades cluster is ∼100 mas yr−1 (Bouvier et al. 2008), which is quite different To determine the spectral types of the VLM candidates, we from that of Galaxy field stars and therefore can be used for exploited the method of luminosity–spectral type calibration by the separation of genuine cluster members from background and Kraus & Hillenbrand(2007). This method is based on a large foreground objects. It was found that the faint Hyades members sample of stellar spectral energy distributions (SEDs) and allows are located within the octant of PM space between PA = 90° us to estimate the spectral type (SpT) of a star using only its and PA = 135° (Bryja et al. 1994) with a convergent point at optical/NIR photometry. The results of this classification are pre- PA ∼ 115°. sented in Table A.1. The second to last column holds the spectral Several all-sky surveys exist in the literature which provide type derived from the TLS I-band magnitudes and the last col- PMs for a large number of stars. Cross-identification of our umn is the average of the three SpT values determined from selected candidates in the USNO-B1 catalogue show, however, 2MASS JHKs. Spectral type sequences based on 2MASS pho- that the given PMs of some objects are questionable because of tometry are self-consistent and the SpT estimates derived from close stellar binarity that may have affected the measurements. these bands agree very well: the SpT values are equal to or lie The same shortcomings are found in the PPMXL, which within one spectral subclass. At the same time, the spectral type combines USNO-B1.0 and 2MASS astrometry. Therefore, we derived from 2MASS is systematically later than those calcu- decided to measure our own precise positions for our selected lated from the TLS I band. Moreover, this difference increases candidates to derive more reliable PM results, especially for the towards later SpT objects. This may imply that our I-band cali- faint objects. bration has some bias with respect to the 2MASS JHKs-system. Therefore, we measured the (X–Y) location of our optically Considering this result in more detail, one can say that the SpT selected candidates on the astrometrically calibrated TLS RI determined from the two methods agree well (with the differ- images and determined their sky coordinates (J2000). To com- ence of a SpT subclass) for most objects whose spectral indices pute the PMs from several epochs, we then cross-referenced were derived from the 2MASS photometry (58 of the 66 objects), our candidates with the following surveys: POSS1 (R), POSS2
A34, page 5 of 14 A&A 611, A34 (2018)
(R + I), 2MASS, UKIDSS K1 band, and WISE; we then obtained Caballero et al.(2007). Our calculation shows that our survey is astrometry for seven independent epochs (TLS RI-averaged) complete to 21.5 in the R band and 20.5 in the I band. The lim- from 1950 to 2010. Most objects fainter than I = 17 could not iting magnitude of the RI survey is about 1.5–2 mag fainter than be detected on the POSS1-R plates. These objects were mea- the completeness magnitude, i.e. the limiting magnitude is 23 for sured on the WISE 3.4 and 4.6 µm bands, where they were all the R band and 22.5 for I. Since we used the same CCD camera, detected, and were small enough to provide high-quality astrom- these values are similar to those of our Coma imaging survey etry. These objects therefore only have an epoch difference from (Melnikov & Eislöffel 2012), but the Hyades (m − M = 3.3) are 1990 to 2010. We note that all objects could be measured on closer to the Sun than the Coma open cluster (m − M = 4.7) and the downloaded 2MASS images, even those that are not in the therefore we can detect objects in the Hyades with an absolute 2MASS catalogue, while UKIDSS covers the Hyades only in its magnitude that is ∼1.5 fainter than in the Coma cluster. K1 band. Combining TLS RI-photometry with 2MASS IR we have The equatorial coordinates were extracted from all photo- selected 66 photometric candidates (Fig.2 and Table A.1) using metric surveys for the same epoch J2000 and thus a change the modern BT-Settl theoretical model (Baraffe et al. 2015; of object positions should only depend on its PM. Finally, the Allard 2016). The photometric candidates span magnitudes from PM of every target was derived from linear fitting the posi- I ∼ 14.5 to 18.7, covering the mass range from 0.15 M to tion changes over the epochs spanned by our data. The typical 0.07 M (67 Jupiter masses). The objects with I = 14.5–17.7 −1 errors of the measurements are σ(µα cos δ) = 14.1 mas yr and correspond to spectral types from M2 down to L0 based on −1 σ(µδ) = 11.2 mas yr for the PMs measured within 1950–2010 spectro-photometric classification from Kraus & Hillenbrand −1 −1 and σ(µα cos δ) = 28.5 mas yr and σ(µδ) = 27.2 mas yr for (2007), whereas the fainter objects (I > 17.7) should have spec- the 1990–2010 epochs. tral types later than L0. At the adopted distance, the boundary One source that can affect our PM results is a visual binary between stellar and substellar objects lies at I ∼ 18.1 in the CMD system. Reid(1993) found that the Hyades may have a substan- (dashed horizontal line) assuming a Hyades age of 625 Myr. tial fraction of the binaries, from 25% to 60% depending on the The PM selection allowed us to discriminate the cluster model. If we measure the position of an unresolved binary, we members from field objects with PM lower or higher than that believe that we derive the motion of the system as a whole, but of the cluster. The PM selection for our photometric set resulted if we measure the motion of an object which is actually a com- in 14 PM members, which are listed in Table1 containing our panion of a visual binary system, the measured PM can differ RI photometry of the objects as well as their PM values. Coor- from that of the system because the components are involved in dinates of the objects extracted from the TLS frames are based motion around their centre of mass. We inspected all selected on the J2000 epoch of the Guide Star Catalog v1.2. Only one member candidates and if they had signs of binarity, we marked PM member (TLS-Hy-7) is located well under the substellar them in Tables1 and A.2 as “VB” for probable visual binaries borderline (I > 18.1) and we classify it as a photometric BD. (visual partially resolved systems) or as “VB?” for wide stellar pairs with lower probability (resolved systems). One can see that 4.1. Comparison with previous studies the binary fraction in our selection set is not as high as predicted by Reid(1993). This is probably due to the source detection Several studies of the Hyades during the last years were focused method used; the method used the shape of stellar images to on improving the census of its lowest mass members. Some of separate single stellar objects from extended sources such as the surveys tried to cover as much area as possible around the galaxies or artefacts. As a result, the many partially resolved sys- Hyades core using new all-sky surveys. For example, the Röser tems whose components were not resolved during the detection et al.(2011) and Goldman et al.(2013) studies are based on process seem to have been excluded. the PPMXL/Pan-STARRS1 surveys and did all-sky searches for The resulting measured PMs for all 66 objects are shown as a Hyades members in a very wide region covering ∼6500 deg2 vector diagram of µα cos δ and µδ in Fig.3. The objects with PMs around the cluster centre (a radius of ∼30 pc). Other surveys were satisfying Hyades membership were selected using the same PM focused on the core area of this cluster trying to register substel- box as in Bouvier et al.(2008) and Bryja et al.(1994). Four other lar members towards the lowest masses, e.g. the Bouvier et al. objects are located outside of this box, but can also be classified (2008) survey. In our survey we used the same approach and tried as Hyades members within their error bars. In total, 14 objects to discover the faintest RI objects in the Hyades core. Neverthe- were selected as Hyades PM members (Table1). Two objects less, our TLS and Bouvier et al.(2008) surveys do not cover the have been selected as possible components of visual binaries same area. A comparison of our survey with the spatial cover- TLS-Hy-2 and -8; both have also been classified as Hyades mem- age of Bouvier’s study (16 deg2) shows that they overlap over bers in previous studies as LH 234 (Leggett & Hawkins 1988) ∼60% (∼13.5 sq. deg), i.e. our imaging survey covers an addi- and RH 230 (Reid 1992). However, both objects are mentioned tional 10 deg2. Thus it is complementary to the Bouvier et al. in these studies as single stars without counterparts. Therefore, study. it is possible that these visual pairs are not physical binaries, but The Röser et al.(2011) and Goldman et al.(2013) surveys were classified as such due to projection effects. The optically both used the same technique of kinematic selection (the con- selected candidates whose PMs do not satisfy Hyades member- vergent point method), and Goldman et al.(2013) published a ship are listed in Table A.2. Their PMs are listed with individual list of 63 additional candidates not included in the Röser et al. rms errors and actual epoch range. (2011) list. Since the upper mass limit of our TLS survey is ∼0.15 M , it does not overlap with the lower mass limit of the Röser et al.(2011) survey ( 0.2 M ). To check this we tried to 4. Results and discussion cross-reference both our PM candidates and photometric candi- dates with the Röser et al.(2011) list, but, as expected, we found Our RI survey covered 23.4 deg2 in the core area of the no common objects. The Goldman et al.(2013) survey has a Hyades (Fig.1). We estimated the completeness and the limiting lower mass limit of 0.1 M so that there could be some common magnitude of the survey in the RI bands as described in sources. However, since the surface density of low-mass Hyades
A34, page 6 of 14 S. Melnikov and J. Eislöffel: The VLM/BDs in the Hyades cluster
Table 1. Hyades PM member candidates.
Object RATLS DecTLS IR − I µα cos δ µδ Epoch Mass Notes −1 TLS-Hy-··· (J2000) (mag) (mas yr )(M ) 1 04 17 31.3 15 23 01 14.88 1.61 95.1 ± 16.6 −97.6 ± 26.0 1950.94–2006.91 0.12 2 04 18 00.5 13 35 58 16.04 2.29 69.1 ± 24.8 −16.5 ± 10.0 1953.78–2006.91 0.09 LH 234, VB 3 04 18 51.1 13 59 24 14.82 1.73 98.1 ± 18.6 −75.0 ± 9.6 1953.78–2006.91 0.13 LH 222n 4 04 18 57.7 16 10 56 15.44 1.73 65.7 ± 18.5 −37.6 ± 17.3 1950.94–2006.91 0.10 5 04 19 41.8 16 45 22 17.00 2.42 88.3 ± 42.9 −94.3 ± 17.8 1995.73–2006.91 0.08 LH 214 6 04 20 50.3 13 45 53 17.32 2.42 80.9 ± 32.8 −8.1 ± 11.4 1989.85–2010.66 0.08 LHD 0418+1338 7 04 22 05.2 13 58 47 18.63 2.46 107.1 ± 37.7 −19.2 ± 3.5 1995.73–2010.66 0.07 Hy 6 8 04 26 19.1 17 03 02 14.93 1.92 102.8 ± 11.5 −27.6 ± 9.9 1955.94–2007.18 0.12 RH 230, VB 9 04 27 05.3 13 10 33 16.44 2.03 83.1 ± 12.9 −4.4 ± 8.1 1955.95–2006.91 0.09 10 04 29 02.9 13 37 59 15.15 2.32 100.9 ± 12.0 −20.8 ± 5.4 1955.95–2006.91 0.11 LH 91,CFHTHy-16 11 04 30 04.2 16 04 08 15.02 2.04 91.3 ± 17.2 −58.8 ± 14.6 1955.95–2006.91 0.12 RH 281, LH 85, CFHTHy-14 12 04 31 16.4 15 00 12 14.69 1.92 104.0 ± 17.9 −16.7 ± 11.2 1955.95–2006.91 0.13 LH 68n, CFHTHy-12 13 04 32 51.2 17 30 09 17.83 2.67 113.3 ± 25.7 −17.5 ± 23.0 1955.94–2006.91 0.08 LHD 0429+1723 14 04 33 28.1 17 29 32 15.56 1.83 62.8 ± 15.6 −35.0 ± 27.0 1955.94–2006.91 0.10
Notes. VB = a visual binary (partially resolved system), cross-identification: CFHTHy = Bouvier et al.(2008); Hy = Hogan et al.(2008); LH = Leggett & Hawkins(1988); LHD = Leggett et al.(1994); RH = Reid(1992). candidates in the survey is quite low in the core region, only 3 JHK-magnitudes of LH 222 are considerably different from of 62 candidates from the Goldman et al.(2013) list are located 2MASS JHK-photometry (>1 mag); this may be due to genuine within the area covered by our TLS fields. Nevertheless, we were variability of this object or may be a false detection. In the case able to cross-identify two objects with this survey: TLS-Hy-8 = of our RI and the 2MASS photometry, a location of LH 222 on 66.5793 + 17.0506 and TLS-Hy-12 (photometric candidate) = both I − (R − I) and NIR CMDs agrees well with the BT-Settl 67.8182 + 15.0034. The third object from the list of Goldman theoretical sequence for the Hyades distance. LH 68 was also et al.(2013) at R = 13.364 was too bright for the TLS survey. selected as a probable member in Bouvier et al.(2008) based on Bouvier et al.(2008) selected 22 low-mass probable mem- the criteria of photometry and PM (CFHTHy-12). bers based on their photometry and PM. Ten of the objects are We have selected only one object (TLS-Hy-7) lying near the brighter than I = 14 and two BD candidates have I > 21.5. substellar domain (M ≤ 0.075 M ) and cross-identified it with Therefore, the magnitudes of these objects were out of our photo- Hy 6 from Hogan et al.(2008). Since the photometric error is metric range. Of the ten objects lying within the TLS magnitude large at these magnitudes, we are unable to ascertain whether range, three objects are located outside of the TLS fields. Among the object is lying above or below this boundary. Thus, this can- the remaining seven objects there are three candidates in com- didate may be a BD or instead the lowest mass stellar member mon: TLS-Hy-10 = CFHTHy-16, TLS-Hy-11 = CFHTHy-14, known. Casewell et al.(2014) observed this object with medium- and TLS-Hy-12 = CFHTHy-12. Four of the objects were not resolution NIR spectroscopy and classified TLS-Hy-7 as a detected because of special observing conditions: CFHTHy-15 Hyades BD member with spectral type of M8–L2. This spectral and -17 are components of a close double source (RHy 240AB, classification agrees with our estimation (M9–L0) obtained from Reid 1992), which was not resolved in our survey photome- the spectro-photometric calibration (Kraus & Hillenbrand 2007). try, and we therefore removed these objects from our lists. The In total, ten previously selected Hyades members are redis- remaining two objects (CFHTHy-18 and -19) are located in the covered in our TLS survey (marked in the “Notes” column in vicinity of very bright stars with spikes and strong halos, and Table1). Thus only four member candidates from our list are therefore the method used was not able to detect and derive pho- not identified in any of the previous surveys and no new BD tometry for these objects. A comparison of our PM values of the candidates have been found. objects in common with those of Bouvier et al.(2008) shows that they are in agreement within the rms errors. 4.2. An infrared object of special interest: TLS-Hy-153 We also searched for our objects in several earlier surveys covering the Hyades core region: Leggett & Hawkins(1988), The object TLS-Hy-153 (RA = 4h16m51s Dec = +13◦ 160 0900, Reid(1992), Leggett et al.(1994), and Hogan et al.(2008). I = 19.45, R − I = 2.72) was selected as a photometric candi- The cross-identification is indicated in the “Notes” column date, and it is located in the BD region on the I − (R − I) CMD. of Table1. In addition to Bouvier et al.(2008), TLS-Hy- Since this object is quite faint, it was not cross-identified with 11 was also identified as a Hyades member in Reid(1992) a 2MASS source, but only with a UKIDSS object. After trans- (RHy 281) and TLS-Hy-10 was selected as a photometric mem- formation from K1 (UKIDSS) to Ks (2MASS) this object was bers in Leggett & Hawkins(1988) (LH 91) and Leggett et al. placed on the I − (I − K) CMD, but it was rejected from the list (1994) (LHD0426+1331); LH 68 (TLS-Hy-12) and 222 (TLS- of photometric candidates due to its inconsistent colour index Hy-3) were marked as non-members (n) in Luyten et al.(1981) (I − Ks = 3.65). Moreover, the derived PM of the object does based on their PMs. However, our PM values calculated over not agree with that of Hyades. TLS-Hy-153 is hardly visible 50 yr of epoch difference agree well with other Hyades mem- on the I-band image of the TLS survey. At the same time, an ber candidates. According to the photometric distance obtained inspection of WISE images shows that this object is quite bright from UKIRT JHK-photometry (Leggett & Hawkins 1988) LH at 3.4 and 4.6 µm (Fig.4) and at UKIDSS K band (2.2 µm). 222 is located outside the Hyades core. However, the UKIRT Moreover, the WISE images reveal a bright companion (marked
A34, page 7 of 14 A&A 611, A34 (2018)
Fig. 4. TLS and WISE images of TLS-Hy-153 region. The IR compan- ion of TLS-Hy-153 (empty arrow) is invisible in the TLS I-band image, but bright in the WISE 3.4 and 4.6 µm bands. This IR object is also visible at the WISE 12.1 µm, whereas TLS-Hy-153 is not visible at the Fig. 5. SEDs of TLS-Hy-153 and its IR companion. The TLS-Hy-153 wavelengths. SED is fitted with a single Planck function corresponding to the flux distribution of a black body with a temperature of 1380 K (dashed line). The fit is based on the best solution for the blue cut-off and the SED by an arrow) with a similar brightness. At 4.6 µm this object slope around the R, I bands at which the stellar object irradiates. The is brighter than at 3.6 µm and it is brighter than TLS-Hy-153 TLS-Hy-153-IR SED is fitted with two Planck functions, with a black (comparing the peak flux of the central pixels). This IR com- body at 800 and 300 K. panion (hereafter TLS-Hy-153-IR) can even be seen at 12.1 µm. This IR object is also detected on the UKIDSS K-band image: in the W3 band is larger than in W1 or W2. Therefore, we sug- J041652.01+131610.0. The angular separation between the two gest that TLS-Hy-153-IR represents an extremely faint object components averaged from the high-resolution UKIDSS K-map (an ultra-cold BD or a planet-like object) surrounded by a lower is ∼1100, which is ∼500 AU at the distance to the Hyades. temperature structure such as a dust envelope or a circumstellar To estimate the temperature of the objects we constructed disk. their SEDs from the available data: the TLS photometry, the There are two possibilities to consider for the location of WISE, and the UKIDSS surveys. The TLS-Hy-153 SED is this binary object. The first is that this is a wide binary sys- based on the five data points measured in the R, I bands; the tem located in the Hyades; however, as noted above, because of UKIDSS K band; and WISE W1, W2; whereas photometry data its faintness the PM of this object could only be obtained over for TLS-Hy-153-IR are available from three WISE bands, W1, the epoch difference 1997–2010 and does not support its clus- W2, and W3 (Fig.5), and from the UKIDSS K band. The fluxes ter membership. On the other hand, the rms of the PM values of the close companion on the W1, W2 images were separated are quite high. Therefore, we repeated the imaging of this area and calculated using PSF-photometry (IRAF.daophot), whereas at the TLS 2 m telescope with a longer exposure (2 × 20 min) the W3 flux of TLS-Hy-153-IR was estimated using aperture in the I band in 2015. The astrometry of the brighter compan- photometry; the fluxes were then converted to magnitudes with ion on these images confirms the previous estimate of its PM zero points provided by the WISE survey. Both objects are well direction, while the fainter companion is still not visible on these resolved on the high-resolution UKIDSS K-map and therefore exposures. However, if the system is indeed a wide physical pair, we used their photometric magnitudes from the survey. the motion of TLS-Hy-153 around the common mass centre may One can see that TLS-Hy-153 shows a maximum bright- affect the PM determination. Therefore, we cannot exclude the ness at 2.2 µm. We estimate the TLS-Hy-153 temperature by cluster membership of these objects. fitting its SED with a Planck function for a black body. If we A second possibility is that we are looking at a young, more assume that the radiation from the R, I band comes from the stel- distant system in the Taurus star-forming region (SFR). Since the lar object, the best solution that fits the blue cut-off and the SED Hyades are located in front of the Taurus SFR, objects in the SFR slope around the bands gives a flux distribution at T = 1380 K. can be mixed up with genuine Hyades members. We note that the For TLS-Hy-153-IR, the fit scaled to the IR emission in K WISE images show a mini-cluster of bright IR objects around and W1–2 corresponds to a Planck function with a temperature the young stellar object (YSO) 2MASS J04220042+1530212 of ∼800 K, whereas the W3 emission fits better with a black (Rebull et al. 2011). Many of the objects are not visible on our body at 300 K. This implies that the estimate of the TLS-Hy- TLS R- and I-band images, but four objects (TLS-Hy-107, -108, 153-IR temperature using a single Planck function may not be -109, -110) have been selected as photometric candidates in this reliable because at these wavelengths different sources with dif- area; however, they were all rejected after the PM selection. ferent temperatures can contribute to the resultant flux. Indeed, Since these objects are projected on the dim area (compar- measurements of the diameter of the TLS-Hy-153-IR image in ing this region with adjacent ones), all the IR objects might different WISE bands show that the FWHM of its stellar profile be members of the Taurus SFR. The TLS-Hy-153 system is
A34, page 8 of 14 S. Melnikov and J. Eislöffel: The VLM/BDs in the Hyades cluster located 2◦.5 to the south-east from this mini-cluster and may still belong to the periphery of the Taurus SFR. For the Taurus dis- tance of 140 pc, the spatial distance between the objects will be very high: 1540 AU. On the other hand, the star density around TLS-Hy-153 is higher than around this IR cluster which means that the extinction is not strong in this area. However, isolated YSOs located near SFRs in an area without signs of dust clouds with strong extinction are a known phenomenon. Therefore, we neglect the interstellar extinction which is probably low in this area. Unfortunately, we do not know a value of the circumstel- lar extinction for this object which might be considerable in the case of YSOs. If we adopt a distance to the SFR as 140 pc (Kenyon et al. 1994) and its age as 2 Myr (Kenyon & Hartmann 1995), according to the BT-Settl model TLS-Hy-153 is a very low-mass substellar object with M ∼ 0.01 M (10 Mjup) and T ∼ 2200 K. We note, however, that all young BDs that have been found in the Taurus dust clouds so far have been found towards areas with high interstellar extinction. Another param- eter which we can compare is the PM. The PM of TLS-Hy-153 Fig. 6. Present-day mass function of the Hyades between 0.05 and −1 −1 (µα cos δ = 16.1 ± 49.4 mas yr , µδ = −8.1 ± 42.9 mas yr ) 3 M . This MF is combined from our new TLS survey, the Bouvier et al. obtained from the short epoch difference coincides within our (2008) survey and the Prosser & Stauffer database (solid line). The rms with the PM of other members of the Taurus region (e.g. dashed histogram corresponds to the data shown in Bouvier et al. 5.8, −19.5 mas yr−1: Grankin 2013; Ducourant et al. 2005). How- (2008). The data derived from our survey added objects to the Hyades ever, it is very unusual to find such an object so far from the MF in the two lowest mass bins, 0.048–0.12 and 0.12–0.19 M . The error core of the Taurus SFR. If we take the position of the dust bars take into account the Poissonian error and the photometric uncer- tainties as well. For comparison, the Pleiades MF (dashed histogram) cloud where many young stars such as DF Tau and DG Tau are h m ◦ 0 adopted from Bouvier et al.(2008) is overplotted. Both the TLS and the located, α = 4 27 , δ = +25 50 , the angular distance between Pleiades MFs are normalised to the mass distribution of the Prosser & ◦ TLS-Hy-153 and this potential birthplace would be ∆α ≈ 3 and Stauffer database as described in Bouvier et al.(2008). ◦ ∆δ ≈ 12.5 . To reach the current place within 2 Myr, the differ- ence in velocity between the object and the mean Taurus SFR should be about 4 and 22 mas yr−1 for α and δ, respectively, agrees with a power law index of α = −1.3, whereas the Pleiades −1 −1 which led to µα cos δ = 1.8 mas yr and µδ = −42 mas yr for show α = 0.6 for the same mass range. This Hyades MF slope TLS-Hy-153. If we take the position of T Tau, which is also has been calculated assuming that the radial distribution of BDs associated with a dust cloud and close to our target, the differ- and VLM stars is the same and equals rC = rBD ' 3 pc. A larger −1 ence in the velocity will be less than µα cos δ = 3.8 mas yr radius of the BD population (rBD = 7 pc) taken on the assump- −1 and µδ = −31 mas yr . Unfortunately, the PM uncertainty for tion of a fully relaxed cluster increases the potential amount TLS-Hy-153 is quite high, which does not allow us to draw a of BDs, but cannot eliminate the difference between the MFs conclusion on its birthplace. Nonetheless, TLS-Hy-153 and the (Bouvier et al. 2008). This finding confirms that the low-mass accompanying IR object might represent an interesting wide sys- MF of the Hyades is much more poorly populated than in the tem: one of the elements may be a BD and the other a planet-like Pleiades cluster, which is much younger than the Hyades. object (or an ultra-cold BD). In order to build an updated PDMF of the Hyades, we com- bined our new results with the stellar mass statistics presented 4.3. Mass function by Bouvier et al.(2008). In order to calculate the masses for our cluster member selection, we used the mass–magnitude rela- The previous studies found that the Hyades are probably a more tionships defined by the BT-Settl 625 Myr model (Allard 2014). massive cluster than the similarly aged open cluster in Coma. Since the isochrones are available for both R- and I-magnitudes, Röser et al.(2011) estimated a cluster tidal radius of 9 pc, which we estimated the masses from both bands independently. A com- is about twice that of Melotte 111 (5–6 pc, Odenkirchen et al. parison of the two estimates shows good agreement, within 0.01– 1998). Within this tidal radius Röser et al.(2011) found 364 0.02 M . Therefore, we averaged the two values (see Table1). stellar systems with the total mass of 275 M . Reid(1992) esti- Figure6 represents the resulting new, more complete Hyades mated the Hyades gravitational binding radius to be as large as PDMF including our data, the compiled published data for M? > ∼10.5 pc, comprising a stellar population with a total mass of up 0.2 M from the Prosser and Stauffer Open Cluster database, to 480 M . and the data from Bouvier et al.(2008). Since ten of our objects The present-day mass function (PDMF) was investigated in were rediscoveries from previous studies, only four of our new detail by Bouvier et al.(2008) based on a large member sam- candidates have been added to update the known PDMF. The ple compiled in the Prosser & Stauffer database (Bouvier et al. TLS member set was extrapolated to the whole cluster area by 2008). This database, combined from many studies, lists more a factor, taking into account the ratio of the covered areas in than 500 probable Hyades members and allowed them to build the TLS and Bouvier surveys. The renormalised numbers have a PDMF spanning a range of stellar masses of 0.05–3 M . been added to the final MF. The error bars are based on the Bouvier et al.(2008) showed that the Hyades and Pleiades mass Poissonian statistics and also take into account the mass mea- functions are similar in shape for masses ≥ 1 M and agree surement errors converted from the photometric uncertainties. with a Salpeter slope (α = 2.35, Salpeter 1955). However, for Only one object in our new sample falls within the lowest mass the lower masses the Hyades MF becomes shallower than for bin (0.048–0.12 M ), whereas three objects have been added to Pleiades and for a range of M = 0.05–0.2 M the Hyades MF the 0.12–0.19 M bin. Adding our sample to the Bouvier et al.
A34, page 9 of 14 A&A 611, A34 (2018) sample makes the mass spectrum a bit flatter over 0.05–0.20 M the Hyades. We identify four new low-mass member candidates, and the difference between the Hyades and the Pleiades MFs is while a further ten stars from our list can be cross-identified still clearly apparent (Fig.6). We should take into account that with objects discovered in earlier studies. No new photometric this bin not only includes BDs, but also the low-mass stellar substellar objects (BD) were discovered for the distance of the members. If our selected objects are genuine cluster members, Hyades core. We rediscovered only Hy 6 (Hogan et al. 2008) the population in the lowest mass bin is more consistent with the as a PM member and classified it as a photometric substellar core radius of 7 pc for BDs in the Hyades cluster (Bouvier et al. object candidate (BD) based on the comparison of the observed 2008), i.e. rC ' 3 pc for the VLM stars and rC ' 7 pc for BDs. CMD with theoretical model isochrones. With our four new can- From the resulting MF over this low-mass range, we find the didates added to the present-day mass function of the Hyades slope with α = −1.1 ± 0.2, which is close to the value from below 0.15 M , the updated mass function is close to that of Bouvier et al.(2008). Bouvier et al.(2008). However, low-resolution spectra of the This result can be explained as a sequence of the contin- objects in the red and near-infrared spectral domain are desir- uing dynamical evolution of the Hyades, which are older than able in order to check their ages, which should coincide with the Pleiades. Agekian & Belozerova(1979) showed that during the cluster age. In the case of a cluster membership, the objects the evolution of an open cluster, its members can escape from should exhibit signs of relative youth, such as Hα in emission the cluster and form an extended halo around it. Comparing the (Bouvier et al. 2008; Melnikov & Eislöffel 2012). shapes of the Hyades and Pleiades MFs, Bouvier et al.(2008) estimated that the Hyades must have lost >90% of their initial Acknowledgements. J.E. and S.M. acknowledge support from the American substellar population (M < 0.08 M ). However, they concluded Astronomical Society under the 2005 Henri Chrétien International Research Grant. This publication has made use of data products from the Two Micron that currently ∼10–15 BDs could still be located within the All-Sky Survey, which is a joint project of the University of Massachusetts and Hyades cluster core, whereas initially the cluster harboured up the Infrared Processing and Analysis Center/California Institute of Technology to 200 BDs. Nevertheless, although we used the wide selection and the United Kingdom Infrared Deep Sky Survey. This research has made use criteria which included the photometric errors and the clus- of the SIMBAD database, operated at CDS, Strasbourg, France, and of the IRAF ter depth and also the potential natural colour dispersion (Reid software distributed by NOAO. 1993), we did not find any new BD candidates and only four VLM candidates were added to the previous list of members. We also note that the area of the TLS survey was 10 deg2 higher References than in Bouvier et al.(2008). Modern numerical simulations of Agekian, T. A., & Belozerova, M. A. 1979, AZh, 56, 9 the Hyades cluster predict that during the dynamical evolution, Allard, F. 2014, in Exploring the Formation and Evolution of Planetary Systems, “evaporated” BDs and other low-mass members will form elon- eds. M. Booth, B. C. Matthews, & J. R. Graham, IAU Symp., 299, 271 Allard, F. 2016, in SF2A-2016: Proc. Annual Meet. French Soc. Astron. Astro- gated tails out of the main cluster core (Chumak et al. 2005). phys., eds. C. 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A34, page 11 of 14 A&A 611, A34 (2018)
Appendix A: Hyades photometric member candidates In Table A.1, we provide a list of 66 optically selected Hyades member candidates inferred from an I − (R − I) CMD that were selected for follow-up based on 2MASS JHKs photometry.
Table A.1. RI- and 2MASS JHKs photometry of Hyades member candidates.
Object 2MASS RATLS DecTLS IR − IJHKs SpT SpT TLS-Hy-··· (J2000) (mag) (mag) 2MASS IJHKs 101 04170259+1244198 04 17 02.6 12 44 20 16.15 ± 0.01 1.95 ± 0.02 14.02 13.45 13.19 M6 M7 1 04173123+1523010 04 17 31.3 15 23 01 14.88 ± 0.01 1.61 ± 0.01 13.17 12.46 12.18 M5 M6 102 04175203+1536240 04 17 52.0 15 36 24 15.65 ± 0.01 1.90 ± 0.01 13.53 12.91 12.64 M5 M6 2 04180046+1335570 04 18 00.5 13 35 58 16.04 ± 0.01 2.29 ± 0.01 14.13 13.54 13.18 M6 M7 103 04182141+1651372 04 18 21.4 16 51 37 14.55 ± 0.01 1.59 ± 0.01 12.79 12.19 11.93 M5 M5 3 04185110+1359240 04 18 51.1 13 59 24 14.82 ± 0.01 1.73 ± 0.01 12.87 12.30 12.00 M5 M5 4 04185767+1610553 04 18 57.7 16 10 56 15.44 ± 0.01 1.73 ± 0.01 13.64 13.03 12.71 M5 M6 104 04190236+1305327 04 19 02.3 13 05 33 18.52 ± 0.05 2.65 ± 0.13 15.36 14.85 14.45 M9 >L0 105 04193697+1433329 04 19 37.0 14 33 33 17.06 ± 0.01 2.36 ± 0.02 14.36 13.67 13.26 M7 M8 5 04194169+1645222 04 19 41.8 16 45 22 17.00 ± 0.01 2.42 ± 0.01 14.43 13.86 13.53 M7 M8 106 04195465+1647274 04 19 54.7 16 47 28 15.43 ± 0.01 1.83 ± 0.01 13.43 12.73 12.41 M5 M6 6 04205016+1345531 04 20 50.3 13 45 53 17.32 ± 0.02 2.42 ± 0.04 14.27 13.56 13.06 M7 M7 107 04211753+1530035 04 21 17.6 15 30 04 17.17 ± 0.01 2.27 ± 0.02 14.42 13.74 13.26 M7 M8 108 04215127+1532560 04 21 51.3 15 32 57 18.42 ± 0.05 2.20 ± 0.09 15.46 14.04 13.46 M8 M8 109 04215218+1519409 04 21 52.2 15 19 42 16.72 ± 0.01 2.06 ± 0.02 14.24 13.47 13.09 M6 M7 7 04220512+1358474 04 22 05.2 13 58 47 18.63 ± 0.03 2.46 ± 0.09 15.50 14.81 14.25 M9 L0 110 04223075+1526310 04 22 30.7 15 26 32 17.23 ± 0.02 2.32 ± 0.04 14.36 13.55 13.08 M7 M7 111 04223593+1402256 04 22 36.0 14 02 25 15.71 ± 0.01 1.86 ± 0.01 13.67 12.96 12.67 M5 M6 112 04223914+1657504 04 22 39.1 16 57 50 15.01 ± 0.01 1.68 ± 0.01 13.07 12.54 12.26 M5 M6 113 04224621+1227080 04 22 46.2 12 27 09 16.08 ± 0.02 1.95 ± 0.03 14.04 13.48 13.13 M6 M7 114 04225357+1308433 04 22 53.6 13 08 44 14.95 ± 0.01 1.74 ± 0.01 12.95 12.22 11.93 M5 M5 115 04232421+1559537 04 23 24.2 15 59 55 17.34 ± 0.02 2.29 ± 0.04 14.65 13.99 13.60 M7 M9 116 04232470+1541451 04 23 24.7 15 41 43 17.28 ± 0.02 2.41 ± 0.04 14.67 14.12 13.79 M7 M9 117 04232781+1702288 04 23 27.8 17 02 29 15.03 ± 0.01 1.74 ± 0.01 13.07 12.52 12.20 M5 M6 118 04233547+1552267 04 23 35.5 15 52 28 14.22 ± 0.01 1.77 ± 0.01 12.21 11.57 11.29 M4 M5 119 04235702+1632458 04 23 57.0 16 32 46 14.16 ± 0.01 1.69 ± 0.01 12.27 11.72 11.44 M4 M5 120 04240478+1424268 04 24 04.8 14 24 27 15.25 ± 0.01 1.76 ± 0.01 13.44 12.87 12.64 M5 M6 121 04240618+1439207 04 24 06.2 14 39 21 14.92 ± 0.01 1.73 ± 0.01 13.11 12.53 12.28 M5 M6 122 04240703+1332409 04 24 07.0 13 32 41 15.80 ± 0.01 2.06 ± 0.01 13.38 12.82 12.51 M6 M6 123 04243380+1529345 04 24 33.8 15 29 36 15.04 ± 0.01 2.20 ± 0.01 12.72 12.05 11.75 M5 M5 124 04243861+1604462 04 24 38.6 16 04 47 15.44 ± 0.01 1.82 ± 0.01 13.63 13.04 12.70 M5 M6 125 04251419+1541079 04 25 14.3 15 41 08 17.70 ± 0.02 2.69 ± 0.04 14.71 13.94 13.49 M7 M9 126 04252314+1735150 04 25 23.1 17 35 15 17.71 ± 0.02 2.43 ± 0.07 14.86 14.27 13.74 M7 L0 127 04253933+1723033 04 25 39.3 17 23 03 15.73 ± 0.01 1.91 ± 0.01 13.75 13.18 12.87 M5 M5 128 04260896+1310379 04 26 08.9 13 10 38 15.59 ± 0.01 1.83 ± 0.01 13.40 12.72 12.38 M5 M6 129 04261090+1408590 04 26 10.9 14 08 57 15.75 ± 0.01 2.05 ± 0.01 13.63 13.14 12.79 M6 M6 8 04261903+1703021 04 26 19.1 17 03 02 14.93 ± 0.01 1.92 ± 0.01 12.87 12.28 11.91 M5 M5 130 04263477+1339146 04 26 34.7 13 39 14 14.98 ± 0.01 1.74 ± 0.01 13.18 12.58 12.28 M5 M6 131 04264240+1638001 04 26 42.4 16 38 00 15.45 ± 0.01 1.76 ± 0.01 13.54 12.97 12.63 M5 M6 132 04270289+1558224 04 27 02.9 15 58 24 14.83 ± 0.03 1.93 ± 0.05 12.90 12.18 11.80 M5 M5 9 04270528+1310323 04 27 05.3 13 10 33 16.44 ± 0.01 2.03 ± 0.01 14.32 13.71 13.36 M6 M8 133 04273932+1507345 04 27 39.2 15 07 34 15.54 ± 0.01 2.23 ± 0.01 13.34 12.80 12.47 M5 M6 134 04284199+1533535 04 28 42.0 15 33 54 14.78 ± 0.01 1.82 ± 0.01 12.88 12.29 12.04 M5 M5 135 04285859+1517386 04 28 58.6 15 17 38 15.07 ± 0.01 1.95 ± 0.01 13.06 12.38 12.08 M5 M6 10 04290287+1337586 04 29 02.9 13 37 59 15.15 ± 0.01 2.32 ± 0.01 12.65 11.94 11.62 M5 M5 11 04300417+1604079 04 30 04.2 16 04 08 15.02 ± 0.01 2.04 ± 0.02 12.88 12.33 11.99 M5 M5 136 04301227+1301086 04 30 12.3 13 01 09 16.48 ± 0.02 2.07 ± 0.03 14.14 13.54 13.16 M6 M7 137 04302653+1737482 04 30 26.5 17 37 48 15.38 ± 0.01 1.75 ± 0.01 13.55 12.92 12.64 M5 M6 138 04302914+1347595 04 30 29.1 13 48 01 14.87 ± 0.01 1.82 ± 0.01 12.84 12.07 11.72 M5 M5 139 04304840+1455210 04 30 48.4 14 55 21 15.10 ± 0.01 1.68 ± 0.01 13.31 12.64 12.33 M5 M6 140 04305333+1725355 04 30 53.3 17 25 36 14.78 ± 0.01 1.76 ± 0.02 13.05 12.41 12.17 M5 M6 141 04310937+1706564 04 31 09.3 17 06 56 15.22 ± 0.02 1.73 ± 0.02 13.37 12.83 12.54 M5 M6 12 04311634+1500122 04 31 16.4 15 00 12 14.69 ± 0.01 1.92 ± 0.01 12.63 12.07 11.71 M5 M5
A34, page 12 of 14 S. Melnikov and J. Eislöffel: The VLM/BDs in the Hyades cluster
Table A.1. continued.
Object 2MASS RATLS DecTLS IR − IJHKs SpT SpT TLS-Hy-··· (J2000) (mag) (mag) 2MASS IJHKs 142 04312630+1329547 04 31 26.3 13 29 55 15.19 ± 0.01 1.73 ± 0.01 13.32 12.78 12.46 M5 M5 143 04315853+1300465 04 31 58.5 13 00 47 15.41 ± 0.01 1.76 ± 0.01 13.31 12.58 12.23 M5 M6 144 04323779+1437237 04 32 37.7 14 37 25 15.49 ± 0.01 1.80 ± 0.01 13.75 13.21 12.81 M5 M7 145 04323801+1508525 04 32 38.0 15 08 52 16.87 ± 0.01 2.33 ± 0.02 13.92 13.14 12.73 M7 M6 13 04325119+1730092 04 32 51.2 17 30 09 17.83 ± 0.02 2.67 ± 0.06 14.69 13.99 13.56 M8 M9 146 04325917+1652587 04 32 59.1 16 52 58 15.11 ± 0.01 1.74 ± 0.01 13.23 12.54 12.24 M5 M6 14 04332808+1729317 04 33 28.1 17 29 32 15.56 ± 0.01 1.83 ± 0.01 13.71 13.15 12.86 M5 M6 147 04333831+1712198 04 33 38.3 17 12 18 14.75 ± 0.01 1.86 ± 0.01 12.81 12.23 11.91 M5 M5 148 04335100+1257033 04 33 51.0 12 57 05 16.11 ± 0.01 2.11 ± 0.02 13.86 13.29 12.95 M6 M7 149 04335913+1738506 04 33 59.0 17 38 50 15.92 ± 0.01 1.98 ± 0.01 13.90 13.35 12.99 M6 M7 150 04341755+1711312 04 34 17.5 17 11 31 15.08 ± 0.01 2.05 ± 0.01 12.88 12.27 11.94 M5 M5 151 04342571+1426147 04 34 25.8 14 26 15 15.88 ± 0.01 1.99 ± 0.01 13.84 13.31 13.01 M6 M7 152 04361002+1447125 04 36 10.0 14 47 12 17.43 ± 0.01 2.49 ± 0.04 14.83 14.20 13.81 M7 L0
Table A.2. Proper motion of Hyades probable non-members.
Object µα cos δ µδ Epoch Notes Object µα cos δ µδ Epoch Notes (mas yr−1) (mas yr−1) (mas yr−1) (mas yr−1) 101 50.6 ± 12.4 −20.5 ± 6.3 1953.78–2010.67 127 22.7 ± 14.7 −11.1 ± 7.6 1955.94–2007.18 VB? 102 38.3 ± 19.7 −58.7 ± 23.4 1950.94–2006.91 128 −3.7 ± 12.0 −0.0 ± 7.2 1955.95–2006.91 103 −16.7 ± 20.5 −82.7 ± 18.5 1950.94–2006.91 129 72.6 ± 12.7 −280.9 ± 6.1 1955.95–2009.69 104 −58.9 ± 13.8 −3.0 ± 33.6 1995.73–2010.66 VB 130 1.2 ± 11.2 −42.9 ± 5.2 1955.95–2009.69 VB? 105 61.1 ± 18.2 13.0 ± 4.6 1953.78–2006.91 131 −2.6 ± 11.8 −14.6 ± 10.4 1955.94–2006.91 VB? 106 51.2 ± 20.6 −36.6 ± 12.3 1950.94–2006.91 132 −3.7 ± 18.9 −8.2 ± 16.6 1955.94–2006.91 107 −42.7 ± 14.8 −6.0 ± 9.1 1955.95–2007.22 133 −93.1 ± 14.3 0.4 ± 6.5 1955.95–2007.22 108 −32.6 ± 25.1 17.9 ± 12.9 1989.85–2010.67 134 −3.1 ± 14.4 −21.7 ± 13.0 1955.95–2007.22 109 −29.0 ± 12.0 −25.4 ± 14.1 1955.95–2007.22 135 79.7 ± 15.2 −126.7 ± 12.0 1955.95–2007.22 110 −48.1 ± 29.8 30.2 ± 17.1 1989.85–2010.67 136 59.6 ± 16.1 −10.3 ± 13.9 1955.95–2006.91 111 40.0 ± 13.1 −25.6 ± 4.5 1955.95–2009.69 137 51.7 ± 11.0 −32.8 ± 15.3 1955.95–2007.18 VB? 112 −6.0 ± 11.2 −30.1 ± 10.0 1955.95–2007.18 138 6.1 ± 15.8 −31.3 ± 9.7 1955.95–2006.91 113 19.5 ± 2.6 −22.6 ± 3.0 1955.95–2010.67 139 1.9 ± 12.2 −10.4 ± 7.5 1955.95–2006.91 114 1.9 ± 13.7 1.0 ± 9.6 1955.95–2006.91 VB 140 −7.4 ± 10.0 −12.3 ± 15.5 1955.95–2007.18 VB? 115 −54.2 ± 43.6 34.9 ± 22.5 1989.85–2010.67 141 31.7 ± 17.5 −93.1 ± 14.4 1955.94–2007.18 VB? 116 −0.8 ± 34.3 −260.9 ± 16.6 1989.85–2010.67 142 −4.6 ± 10.7 −20.3 ± 6.5 1955.95–2006.91 117 5.7 ± 12.4 −22.8 ± 8.3 1955.95–2007.18 143 −3.9 ± 9.2 −5.2 ± 7.5 1955.95–2006.91 118 −5.0 ± 14.5 −15.5 ± 16.6 1955.95–2007.22 VB 144 6.1 ± 13.1 110.1 ± 9.9 1955.95–2010.67 119 −1.4 ± 13.1 −17.9 ± 11.5 1955.95–2006.91 145 −5.6 ± 14.1 12.8 ± 10.3 1955.95–2006.91 120 −42.9 ± 11.9 36.1 ± 4.5 1955.95–2010.67 146 −5.0 ± 15.4 −5.3 ± 22.4 1955.94–2006.91 121 −6.8 ± 15.3 41.3 ± 4.8 1955.95–2007.22 147 −23.1 ± 15.3 −114.4 ± 20.2 1955.94–2006.91 122 4.4 ± 14.9 −4.4 ± 6.1 1955.95–2009.69 148 19.2 ± 21.4 −29.7 ± 7.0 1955.95–2006.91 123 4.2 ± 13.8 −40.4 ± 16.8 1955.94–2007.22 VB? 149 −17.0 ± 4.0 −6.7 ± 1.0 1955.95–2006.91 124 20.9 ± 12.9 −36.6 ± 13.1 1955.94–2008.84 150 7.4 ± 11.1 −18.5 ± 21.8 1955.95–2006.91 125 22.5 ± 9.8 −87.5 ± 7.6 1955.94–2009.68 VB? 151 204.7 ± 10.1 12.9 ± 6.3 1955.95–2010.67 126 −4.4 ± 7.6 −21.0 ± 7.4 1995.73–2010.67 VB? 152 −112.3 ± 19.4 −66.2 ± 10.7 1989.94–2010.68 VB?
Notes. VB = a visual binary (partially resolved system), VB? = a possible visual binary (resolved system).
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